Important gaps remain in our understanding of the effects of modifiable and non-modifiable factors on bone development during late adolescence/early adulthood, and on the impact of early bone health on the risk for common adult conditions such as osteoporosis and bone fractures. Few studies have longitudinally assessed bone accrual and changes in bone geometry at this life stage. The Iowa Bone Development Study (IBDS), in its 13th year, is a longitudinal investigation of the effects of behaviors, selected physical attributes, and genetic factors on bone development in a childhood cohort first examined at five years of age. Our goal here is to increase understanding of bone development during the first two decades of life and determinants of bone health by extending our assessments into early adulthood (age 19), thus allowing a comprehensive assessment of bone development from ages 5 to 19. We have assessed bone measures using dual energy x- ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT) for our cohort at 5 (n=470), 8 (n=539), 11 (n=483), and 13 (n=489) years of age;age 15 assessments are nearly complete (n=407 to date);and age 17 exams have been initiated (n=110). Fluoride intake and dietary data have been collected from birth;periodic physical activity assessments have been made by accelerometry and questionnaire;and variants in candidate genes involved in bone mineralization, matrix structure, and growth have been typed. We have analogous cross-sectional measures for parents of cohort members. In this revision/supplement, we will collect age 19 bone outcomes (DXA and pQCT), fluoride intake and other dietary, alcohol, tobacco, physical activity, maturity, and body composition data, conduct computational mechanics modeling, and conduct high resolution multi-detector CT scans of the distal tibia in a large subset(n=160), providing unique information concerning trabecular bone architecture. We propose to assess and define the effects of the following on bone mass and geometry at age 19 and bone development longitudinally from age 5 to 19: 1) the time-specific and cumulative effects of fluoride intake;2) the time-specific and cumulative effects of behaviors (dietary patterns, alcohol intake, tobacco use, and physical activity) and physical attributes (gender, growth and development, body composition, and muscle strength);and 3) the time- specific and cumulative effects of behaviors, physical attributes, and genetic factors, as well as interaction effects (environment x environment, gene 4 environment, and gene 4 gene), building on the results obtained by addressing #1 and 2 above. and 4) Assess age 19 tibial volumetric bone mineral and novel architectural parameters using high resolution multi-detector CT and describe associations with explanatory factors. Continued follow-up of this cohort to early adulthood will lead to major insights into the relative importance of these factors on bone development and the times of greatest impact, providing a strong foundation for designing the best strategies and future intervention trials to optimize skeletal health. PUBLIC HEALTH RELEVANCE: Although osteoporosis is generally considered a disease of older adults, there is increased recognition of the importance of adequate bone accrual in childhood, adolescence, and early adulthood for prevention of bone disease later in life. However, many gaps remain in our understanding of how bone develops during childhood/adolescence/early adulthood, including the effects of modifiable and non-modifiable factors. Continued follow-up of the Iowa Bone Development Study cohort to age 19 provides a unique opportunity to assess the relative importance of fluoride and other dietary, physical activity, body composition, and genetic factors on bone development, maturation and bone properties to near peak bone mass. Findings will improve our understanding of the best strategies to optimize skeletal health and prevent future bone disease.